The report, released in conjunction with today's Senate committee hearing on dietary supplements and the elderly, revealed the results of an investigation in which GAO investigators posed as elderly customers to ask sales staffers at 22 storefront and mail-order retailers about herbal dietary supplements. The GAO also reviewed marketing claims made on 30 retail websites.

Investigators found that sales people gave them "potentially dangerous advice," such as suggesting that they could take supplements instead of prescription medication, according to the report.

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Illegal Claims to Sell Dietary Supplements?

"What a lot of people do is ... they start taking both at the same time and then slowly you will stop taking the other prescription medicine and just continue with this," said one seller on an audio recording made by the GAO.

A seller during another GAO sales call said that a combination supplements can prevent or cure serious conditions.

"My mom and dad both have high cholesterol and problems maintaining blood pressure; I don't have either one, but my parents have both. If I wouldn't have started taking my product ... I would eventually get it, because it's in my genes," the seller told the GAO.

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Government Sting on Herbal Supplement Sellers

When the GAO repeated the false information back, the seller once again claimed it was true.

In some cases, the promises were not just false, they were dangerous. One herbal supplement seller said that a customer could take Ginkgo biloba with aspirin to improve memory.

"In making these claims, sellers put the health of consumers at risk," Greg Kurtz, head of the GAO Forensic Audits and Special Investigations unit, said in remarks prepared for the committee hearing.

The GAO also listed what it called deceptive claims made by herbal supplement marketers tied to Ginkgo biloba and other popular supplements as ginseng, including assertions that the latter can cure cancer and that the former can treat both impotence and Alzheimer's disease.

Steve Mister, the president and CEO of the Council for Responsible Nutrition, a trade association for supplement manufacturers and ingredient suppliers, defended the industry in an interview with ABCNews.com.

"We understand that there are going to be examples of a few industry outliers who may be violating the law, but that is certainly not representative for the vast majority of the industry which is abiding by the law," he said.

Mister, who said Tuesday that he had not yet seen the GAO report, was scheduled to testify at today's hearing.

Supplement Industry Says It Polices Itself

Mister said that the council has committed $1.5 million for an eight-year program at the Better Business Bureau's National Advertising Division to scrutinize supplement advertising and issue decisions on when supplement sellers should change or pull advertising claims. When a company ignores a National Advertising Division decision, the division refers the ads in question to the Federal Trade Commission.

"This is a good example of industry trying to police itself," he said.

But Mister said the council also supports the FDA's oversight of the industry. It has repeatedly asked, he said, for the Food and Drug Administration to be provided with more resources so it they can enforce a 1994 law that allows the FDA to remove a supplement from the market when the agency can show that the product is not safe.

Claims for Gingko, Ginseng and Garlic Discredited by GAO

The GAO found cases of deceptive marketing or questionable claims tied to Ginkgo biloba, ginseng, garlic and chamomile, according to Kurtz's testimony. The GAO did not disclose the names of the herbal supplement marketers it investigated, but Kurtz said that "all cases of deceptive or questionable marketing and inappropriate medical advice" have been referred to the Food and Drug Administration and the Federal Trade Commission.

The GAO said it found herbal supplement sellers making the following deceptive claims:

Claim: Garlic prevents obesity and diabetes and cures cardiovascular disease. GAO Comment: NIH (National Institutes of Health) does not recognize this herbal supplement as a treatment for obesity, diabetes, or cardiovascular disease.

Claim: Ginseng cures diseases, including cancer. GAO Comment: NIH specifically recommends that breast and uterine cancer patients avoid this product, as it may have an adverse interaction with some cancer drugs.

Claim: Garlic can be taken in lieu of prescribed high blood pressure medication. GAO Comment: Evidence that this product reduces high blood pressure is unclear, and both NIH and FDA state that no dietary supplement can take the place of prescribed medicines.

Claim: Ginkgo biloba can be taken with a daily aspirin prescription. GAO Comment: Taking this product with aspirin may increase the risk of bleeding.

Trace Amounts of Metals, Including Lead, Found in Supplements

The GAO also found trace amounts of lead, mercury, cadmium, arsenic or pesticides in 37 of the 40 herbal supplement products tested. But the level of the contaminants in the products, the GAO report said, did not exceed FDA or Environmental Protection Agency regulations on dietary supplements.

Mister said that plants grown in the ground -- whether they're in a grocery store's vegetable aisle or used as a supplement ingredients -- absorb trace amounts of what's in the soil beneath them. As long as the amount of contaminants present in supplements isn't enough to cause safety hazards, there's no reason to label them, he said.